[P2] Keyboard input mapping module

Centralize key parsing and action mapping so widgets don't each implement their own.

Must handle: arrows, enter, esc, tab, shift-tab, page up/down, home/end, common shortcuts (/, q, ?).

**Deliverable:** `src/keyboard.sh` with a `shellframe_key_read` function and named key constants.
**Effort:** M (~half day) | **Phase:** 2